The immediate goals for the research project during the current year are threefold. (1) It is planned to examine the potentially aggression-inducing properties of chronic administrations of THC. Particularly, the influence of dose level, housing conditions, and animal strain are examined. (2) An important goal for the current and next year is a comparison of the effects of THC and other drugs in three different animal species, including mice, rats, and monkeys. (3) Biochemical investigations will be performed in animals having been subjected to specific acute or chronic THC regimens and having shown significant changes in aggressive behavior. Our results confirm that acutely administered THC suppresses several kinds of aggressive behavior in a number of animal species, including primates, and establish THC, in contrast to most other psychotropic drugs, as an agent with potent and rather specific anti- aggressive properties. The major source of controversial information are studies on the potentially aggression-inducing effects of chronically administered THC in rats. At present, on the basis of studies employing a wide range of doses and different housing conditions, in four different strains of rats, we are not able to substantiate the reports that chronic THC induces aggressiveness in a significant number of animals. However, in some strains of rats, chronic THC initiates indiscriminate and non-specific mouse killing in more than 2/3 of the animals. Further studies will have to establish how elusive or ubiquitous the aggression inducing properties of chronically administered THC are.